Factors that modify tissue responses to chemical carcinogens in different organ systems are studied to evaluate their contribution to changing susceptibilities to carcinogenesis in certain tissues during prenatal and postnatal development. Tissues and organ systems currently under study include the nervous system, kidneys, liver, and intestinal mucosa. Modifying processes now being studied include excision repair of DNA, transport of carcinogen into and within cells, and the changing cellular susceptibility to carcinogens during different stages of the cell cycle. Factors investigated in the past, and anticipated for future study, include immune responses to tumors, the induction and genetics of enzymes metabolizing carcinogens, and tumor promotion.